Isolated switching converters are widely used in off-line power supplies and applications with high security requirement. Precise control strategies with high efficiency, low EMI and low cost are required in these applications. And more, to achieve accurate regulation of the power provided to the load, feedback is necessary.
FIG. 1 illustrates a traditional isolated switching converter 100 which utilizes an auxiliary winding for feedback. When the rectifier D1 is on, the voltage across the auxiliary winding of the transformer T1 is proportional to the output voltage Vout of the switching converter. So, feedback information can be gained through detecting the voltage across the auxiliary winding. As shown in FIG. 1, the voltage across the auxiliary winding is detected to provide a feedback signal FB. The controller generates a control signal DRV1 to control the main switch M1 based on the feedback signal FB and a current sensing signal CS indicative of the primary current Ipri.
The feedback method shown in FIG. 1 is simple and easy, however, it still has some drawbacks. First, the feedback signal FB can indicate the output voltage Vout only when the rectifier D1 is on. As a result, even under no load condition (the output current lout is almost zero), the main switch M1 still needs to be turned on, so the rectifier D1 can be turned on to provide the feedback information. The no load efficiency of the switching converter 100 is limited.
Secondly, under light load condition (the output current lout is smaller than a predetermined threshold), the switching converter 100 often works in DCM (discontinuous current mode) and the switching frequency is low. FIG. 2 illustrates waveforms of the isolated switching converter 100 under light load condition. As shown in FIG. 2, after the energy stored in the transformer T1 has all been transferred to the load, which means the secondary current Isec has been reduced to zero, the feedback signal FB is maintained zero and cannot indicate the load information. If the output current lout is suddenly increased (e.g. at t1), the switching converter 100 cannot give a timely response and there will be a large undershoot at the output voltage Vout.